As an energy source replacing fossil fuel, attention is being focused on solar cells capable of converting sunlight into electric power. Some solar cells and thin-film silicon solar cells in which a crystalline silicon substrate is used are currently becoming commercially available. However, the former is disadvantageous in that manufacturing costs of the silicon substrate are high, and the latter is disadvantageous in that the need to use various types of semiconductor production gases and complicated equipment increases manufacturing costs. Therefore, efforts are being made to reduce a cost per power output in any solar cell by increasing photoelectric conversion efficiency, however, the above-mentioned disadvantages have not been solved.
Japanese Patent No. 2664194 (Patent Document 1) proposes, as a new type of solar cell, a wet solar cell to which light-induced electron transfer of a metal complex is applied.
This wet solar cell includes two glass substrates, each having an electrode on a surface, and a photoelectric conversion layer made of a photoelectric conversion material with a light sensitizing dye adsorbed therein to provide an absorption spectrum in a visible light region and an electrolyte material is held between the electrodes.
When light is irradiated onto the above-mentioned wet solar cell, electrons are generated in the photoelectric conversion layer, and the generated electrons travel through an external electric circuit to one of the electrodes, and the traveled electrons are transported to a counter electrode by ions in the electrolyte to return to the photoelectric conversion layer. With such a sequential flow of electrons, electric energy is drawn out.
However, the dye-sensitized solar cell disclosed in Japanese Patent No 2664194 (Patent Document 1) has a basic structure in which the electrolyte solution is injected in between the opposed glass substrates, each equipped with a transparent conductive film, which is difficult to apply to a large-area solar cell, such as one measuring 1 m per side, although a small-area solar cell can be prototyped. In other words, increasing one solar cell in area increases the generated electric current in proportion to the area, however, a voltage drop in an in-plane direction of a transparent conductive film used for an electrode portion increases, which in turn increases the inner series resistance as a solar cell. This in result raises a problem in that FF (fill factor) in current-voltage characteristics during photoelectric conversion and further a short-circuit current are decreased, resulting in degraded photoelectric conversion efficiency.
WO 97/16838 pamphlet (Patent Document 2) proposes a dye-sensitized solar cell module in which a plurality of dye-sensitized solar cells are arranged in series connection on a single glass substrate equipped with a transparent conductive film.
In this dye-sensitized solar cell module, each dye-sensitized solar cell has a structure in which a porous semiconductor layer (porous titanium oxide layer) to be a photoelectric conversion layer, a porous insulation layer (intermediate porous insulation layer), and a counter electrode (catalyst layer) are laminated sequentially on a transparent substrate (glass substrate) with a transparent conductive film (electrode) patterned into strips, and the transparent conductive film of one dye-sensitized solar cell and the counter electrode of an adjacent dye-sensitized solar cell are arranged to come into contact with each other, and both the solar cells are connected in series.
However, a problem arises in that, when forming the catalyst layer on the porous insulation layer by vapor deposition or the like in the step of producing the dye-sensitized solar cell having the module structure disclosed in WO97/16838 pamphlet (Patent Document 2), if particles constituting the catalyst layer have a low film strength, then, the film is stripped from the catalyst layer when the counter electrode conductive layer is formed thereon, so that a solar cell cannot be produced.
Japanese Patent Laying-Open No. 2002-367686 (Patent Document 3) discloses, in a dye-sensitized solar cell module of an integrated structure having a transparent conductive film, a porous semiconductor layer, a porous insulation layer, and a catalyst layer on a transparent substrate, a technique for providing a porous semiconductor layer made of fine particles having an average particle size smaller than that of fine particles constituting the catalyst layer to prevent the fine particles of the catalyst layer from passing through the porous semiconductor layer to reach the conductive layer, thereby avoiding an internal short-circuit.
As described in Japanese Patent Laying-Open No. 2002-367686 (Patent Document 3), when the porous insulation layer is subjected to screen printing through use of a paste of a catalyst material having high flowability, particles constituting the catalyst layer and particles constituting the porous semiconductor layer need to be controlled in particle size because the catalyst material infiltrates into the porous semiconductor layer to cause an internal short-circuit. However, a problem arises in that particle size control to a great degree leads to degraded performance because the particle size of particles of the porous semiconductor layer significantly affects performance.